As SyzygyΣE said, there is a way, but no, there is no obvious way.
Here's what I said to SyzygyΣE in response to his suggestion on the Steam forum for an easier way to do this:
This is a good idea that's been suggested a handful of times before.
It is on our to-do list currently, though I imagined it as being able to add the moon (or any other body) on its parent's orbital plane. But simply being able to set it after it's already added, like the Auto Orbit button, might be a more simple solution. I'll suggest this. Thanks!